1. Field of Invention
This invention relates to a surface luminous source panel to be used for a surface luminous source apparatus that finds applications in a variety of illuminations including back illuminations using liquid crystal back lights, display stands and display panels.
2. Prior Art
There have been proposed surface luminous source apparatus provided with a light source arranged on an end surface of a surface luminous source panel that can replace conventional luminous source apparatus of the type having a large number of fluorescent light sources arranged on the back side of an irregular reflector panel.
While such known surface luminous source apparatus can be realized in a very thin and compact form, they have the disadvantage of being incapable of providing evenly distributed brightness. In an attempt to eliminate this drawback, Japanese Patent No. 57-128383 proposes an irregular reflection layer formed by white paint and having a reflectivity that smoothly augments with the distance from the surface and Japanese Patent No. 60-205576 discloses a front irregular reflection layer of a transparent substrate formed by white paint to which glass beads are added, while Japanese Patent No. 62-902 teaches a multi-layered structure of surface luminous source panels each provided with an irregular reflection layer.
Since none of these completely eliminate the problem, the inventors of the present invention proposed in Japanese Patent Application No. 63-208670 an irregular reflection layer carrying on it a large number of irregular reflector specks and arranged on a surface of a transparent substrate, wherein the area of the irregular reflector specks of the irregular reflection layer is smoothly increased with the distance from the light source along the layer. The irregular reflector specks are realized in the form of so many dots or strips arranged at a rate of 30 to 65 rows per centimeter by screen printing using a low concentration ink containing solid by 13 to 30 wt % solids in order to produce a low light transmittivity. When an irregular reflection layer according to the above cited invention is used with 8W fluorescent lamps arranged at its opposite end surfaces to form a surface luminous source panel of A4 size, it produces a brightness as high as 2,000 nit.
While a surface luminous source panel such as proposed by the inventor of the present invention and described above can be used for a surface luminous source apparatus having a variety of practical applications, it is still accompanied by the drawback of producing an undesirably highly bright region near the sources of light when it is used as a liquid crystal back light of a portable word processor, computer or television set.
The region showing an undesirably high brightness of such a surface luminous source apparatus generally has a curved contour, which becomes flat or even straight when the irregular reflection layer extends very close to the edges of the transparent substrate.
Although such a surface luminous source panel provides a practically even distribution of brightness, it has a slight unevenness of brightness which can be emphasized to damage the overall effect of the panel particularly when it is used in an apparatus where the evenness of brightness is rigorously required.
It is therefore an object of the present invention to provide a surface luminous source panel which is free from the above problem and ensures a highly even distribution of brightness to meet any rigorous requirements.